An aluminum or bronze powder is used as the pigment for imparting a metallic gloss to writings so as to obtain writings of a metallic color such as gold or silver. The particle diameter of the bronze powder having a specific gravity of 8.5 must be several .mu.m in order to obtain a writing of a metallic color and, therefore, problems such as sedimentation and separation of the powder and clogging at the pen point are caused to make the use of the bronze powder as the pigment for a writing ink very difficult. On the contrary, the aluminum powder having a specific gravity of 2.5 is more easily usable than the bronze powder for producing a writing ink. However, the specific gravity, 2.5, of the aluminum powder is by no means low. Inks for a marking pen containing the aluminum powder as the metallic powder pigment are described in Japanese Patent Publication Nos. 62-37678 (1987) and 1-56109 (1989).
The Japanese Patent Publication No. 62-37678 (1987) discloses a double coloring ink composition comprising a powdery metal pigment such as aluminum powder, an oil-soluble dye, a resin and a solvent, which has an outline effect realized by the penetration and diffusion of the dye around the writing formed by the powdery metal pigment. The Japanese Patent Publication No. 1-56109 (1989) discloses a metallic ink for a marking pen, which comprises a fine metal powder such as a surface-treated aluminum powder, a resin and a solvent, enables smooth delivery from the marking pen, and is easily dispersible at the time of use.
In the ink for a marking pen described in the above patent documents, the sedimentation and separation of the aluminum powder in the ink cannot be inhibited, because the ink has a relatively low viscosity. Thus, such an ink cannot be used unless it is put in an ink container of a writing utensil together with a stirring member such as a metallic ball and the utensil is shaken at the time of use to redisperse the sedimented powdery metal pigment.
Also proposed is a metallic ink for a ball-point pen which can dispense with any stirring member such as a metallic ball unlike the above-described ink for a marking pen.
Japanese Patent Laid-Open No. 60-186573 (1985) discloses an ink capable of realizing a metallic color, which comprises a solvent, a thickening resin soluble in the solvent, a powdery metal pigment and a coloring pigment each in an at least necessary amount and which has a viscosity higher than a necessitated level. It is described therein that since the sedimentation of the metal pigment is only slight in the ink having a high viscosity, it is suitably used for a pressure-type ball-point pen. In addition, Japanese Patent Publication No. 64-4551 (1989) discloses a gelled ink.
Japanese Patent Laid-Open No. 1-210478 (1989) discloses an aqueous metallic ink which is prevented from being stained by pin holes upon ink-application and which comprises a resin, an aluminum paste and water as the main components and an acetylenic alcohol derivative as an additive.
However, the above-described inks proposed in the prior arts are different from an aqueous metallic ink usable for ball-point pens.
Namely, the inks described in the Japanese Patent Publication Nos. 62-37678 (1987) and 1-56109 (1989) and Japanese Patent Laid-Open No. 60-186573 (1985) are oily inks, and the ink compositions described in the Japanese Patent Publication Nos. 62-37678 (1987) and 1-56109 (1989) are directed to a marking pen. The term "marking pen" as used herein refers to one containing a stirring member such as a metal ball in the ink container so that the sedimented aluminum powder can be redispersed by shaking the pen when it is used. In other words, these ink compositions for a marking pen are ones wherein the aluminum powder sediments in a short period of time.
The ink described in the above Japanese Patent Laid-Open No. 60-186573 (1985) is to be used for a ball-point pen having a pressure-type structure, and practically satisfactory results cannot be obtained when it is used for a ball-point pen having a cylindrical ink container usable under atmospheric pressure. The ink disclosed in the Japanese Patent Publication No. 64-4551 (1989) is also an oily ink. As described therein, this ink is nonflowable and, therefore, even though it is delivered in the form of sol by the rotation of the ball at the pen point, continuous writing is difficult. This ink has, therefore, a problem in practice.
The ink disclosed in the Japanese Patent Laid-Open No. 1-210478 (1989) is one developed for preventing staining by pin holes upon ink-application and this specification does not disclose the properties of the ink to be exhibited when it is used for a ball-point pen.
Thus, in designing an aqueous metallic ink containing an aluminum powder as a colorant and usable for a ball-point pen, it is indispensable to use a water-soluble resin to attain such an appropriate ink viscosity as to allow the aluminum powder to be used without redispersion. In addition, it is also indispensable to select suitable properties so that a suitable amount of the ink can be transferred from the ball surface of the ball-point pen to the surface of a substrate such as paper by the rotation of the ball and that the writing is free from bleeding and blurring.